Among electrolytic processes, chlor-alkali electrolysis has certainly the highest industrial interest. Briefly, by this method, the starting reactant, consisting of an aqueous solution of sodium chloride (hereinafter called brine), is separated into gaseous chlorine, an aqueous solution of sodium hydroxide and hydrogen gas by applying electric current, which, in a way, acts as a further reactant. Chlor-alkali electrolysis is carried out today by means of three alternative technologies, i.e. mercury cathode, porous diaphragm and ion-exchange membrane. This last one is the most modern technology and is characterized by low energy consumptions and lack of both environmental and health drawbacks.
Of the first two, the mercury cathode technology is probably bound to a rapid decline in consideration of the strict regulations existing in all the industrial countries in regard to the release of mercury in the environment. The improved cell design allows for meeting the severe tolerance limits imposed by the law, but the public opinion nowadays clings to an "a priori" rejection of any process which may lead to a possible release of heavy metals in the environment.
At present, the diaphragm technology relies on two alternative processes based on the use of diaphragms made only of asbestos fibers or comprising also an organic binder, respectively. The diaphragms made of asbestos fibers only are negatively affected by a poor mechanical stability. As far as it is known today, asbestos fibers may be removed by the erosion caused by the chlorine bubbles with the consequent deterioration of the diaphragm and the sharp and risky increase of the hydrogen content in chlorine. Therefore, in the prior art, efforts have always been pursued to ensure a sufficient distance between the diaphragm and the anode surface in an attempt to minimize the erosion effect. Consequently, cells provided with diaphragms made only of asbestos fibers cannot exploit to the best the advantages offered by the expandable anodes available on the market. As is well known, said anodes have the shape of a flat box wherein the major surfaces, independent from each other, are mobile being provided with expanding means; that is elastic supports capable of exerting a spring action. During assembly, when the anodes are inserted in the limited space between each couple of adjacent cathodes, they are maintained in the restrained position by means of suitable retainers. Once the anodes are positioned, the retainers are removed and the major surfaces of the anodes are pushed by the expanding means towards the opposite surfaces of the diaphragms deposited onto the cathodes. As a general rule, if the expanding means are correctly dimensioned, the anodic surfaces come in contact with the diaphragm surfaces, attaining a configuration known in the art as "zero-gap". In this configuration, the distance between anode and cathode is reduced to the minimum, and therefore, also the cell ohmic drop is minimized, which results in a remarkable energy saving for the electrolysis process. However, in the case of cells provided with diaphragms comprising asbestos fibers only, this advantage must be given up as the cell voltage tends to increase with time, and the diaphragm is often damaged by the intense erosion due to the gas bubbles evolved just on the diaphragm surface. Therefore, with diaphragms made of asbestos fibers only, so far expandable anodes could not be used and a fixed gap is maintained, usually in the range of 6-10 min. Obviously, in this way, the ohmic drop in the electrolyte is higher than in the zero-gap configuration, and therefore, the energy consumption is higher.
A prior art solution to the drawbacks of the diaphragm consisting only of asbestos fibers, comprises adding to the fibers a suitable corrosion-resistant polymeric binder, such as polychlorotrifluoroethylene or the like. These diaphragms, known as modified diaphragms, exhibit a remarkably higher mechanical stability and might certainly operate with a zero-gap configuration with their surfaces in contact with the anodic surfaces. However, the results in terms of cell voltage obtained with anodes in the zero-gap configuration and modified asbestos diaphragms are not as satisfactory as expected. In fact, the cell voltage, which should decease regularly as the gap between the anode and the diaphragm decreases, reaches its minimum at a distance in the range of 3.5-4 mm (see J. W. Winingsand D. M. Porter in Modem Chlor-Alkali Technology, 1980, pages 30-32). Presumably this effect is caused by the presence of the polymeric binder which tends to give a certain hydrophobicity to the diaphragm on whose surface, therefore, the chlorine gas bubbles tend to stick, the more when the bubbles are numerous; that is the more the surface of the diaphragm is dose to that of the anode. The bubbles sticking to the diaphragm represent a barrier to the passage of current, and this explains why, below a certain critical gap (3.5-4 mm as aforesaid), the cell voltage does not decrease anymore as the gap decreases, but on the contrary, it tends to increase (bubble effect).